This invention relates to a method and an apparatus embodying the method for producing electric wires (for example, such as insulated cores for communication cables) having an insulating layer of plastic foamed on the periphery of the conductor.
For prior art methods of producing foamed plastic insulated wires using an extruding machine, one method has been proposed wherein, for example, a foaming agent inlet is provided in a part of an extrusion cylinder or an extrusion screw in the metering portion of the cylinder. As is well known, the metering portion of an extrusion cylinder is that portion which stabilizes the temperature, pressure and volume of the resin so that a uniform amount of resin is extruded. This method, however, has the disadvantage that it is extremely difficult to mix the foaming agent into the thermoplastic resin. This is due to the fact that the resin pressure in the metering portion is very high (generally, in excess of 150 kg/cm.sup.2), and as is well known, the greater the pouring pressure of the foaming agent as compared to the resin pressure the greater will be the transmission coefficient (i.e., the degree of mixing).
In another example of a known method, a gaseous foaming agent is introduced into a breaker plate at the rear of the metering portion. In this case, also, it is difficult to dissolve the gas into the resin for the same reason noted above. Furthermore, in the latter case, the resin and gas are agitated by means of a screw type agitator in a cross head located upstream of the breaker plate portion to thereby greatly vary the extrusion pressure, with the result that it has been difficult to continuously obtain uniform foamed bodies.
A further method of obtaining a foamed body is carried out by directly admitting under pressure a foaming agent into an extruding machine and using a screw having a compression portion with a volume surrounded by one pitch reduced in the direction of extrusion, as in a slow compression type screw. This method possesses a disadvantage such that the poured foaming agent is difficult to mix or dissolve in the resin, and as a result, it is difficult to obtain uniform foamed bodies.
It is therefore an object of this invention to provide a method and an apparatus embodying said method for producing electric wires, while eliminating the various defects noted above, wherein a foaming agent may very easily be mixed and dissolved in a resin even if the pouring pressure of the foaming agent is low, and variation of extrusion pressure can be minimized.